A simple and sensitive photothermal technique -photothermal detuning (PTDT), which is based on the absorption-induced shift of reflectance or transmission spectrum of an optical coating, is developed to measure the absorption of coated optical components. A PTDT theory is developed to describe the signal's dependence on the structural parameters of the optical coatings and on the geometric parameters of the experimental configuration. An experiment is performed to measure the PTDT signal of a highly reflective multilayer coating used in 532nm by using a probe beam with a wavelength of 632.8nm. By optimizing the incident angle of the probe beam, the measurement sensitivity is maximized. Good agreements between the theoretical predictions and experimental results are obtained.